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The present invention relates generally to fiber optic couplers and in particular to fused/tapered couplers made from optical fibers with photosensitive cladding regions.
Fiber optic couplers have many uses, such as multiplexers, optical filters, and add/drop couplers. These couplers have one or more input optical fibers, and one or more output optical fibers depending on the configuration and function of the coupler.
A typical optical fiber used in these couplers consists of a core region coaxially surrounded by a cladding layer. The core region is usually some form of SiO2 and may be doped with known dopant elements to render the core photosensitive. A photosensitive core may be used as a fiber amplifier, an optically pumped laser, or to have a fiber Bragg grating written within the fiber by a UV laser or other source. Optical power (light) is propagated through the core of the fiber by total internal reflection, where the index of refraction of the core is larger than the index of refraction of the cladding. The cladding, in general, is not photosensitive. A core may be either a multi-mode core, allowing many different modes of propagation through the optical fiber, or a single mode core, allowing a single mode of propagation through the optical fiber.
Although optical fibers provide greater bandwidth and less attenuation than electrically conductive wires, a signal is not propagated without loss or distortion. Typical losses in a fiber may include attenuation and distortion caused by dispersion of the signal within the core is also a factor. In some cases the attenuation of the signal is increased by optical power that has been coupled to the cladding and stays within the cladding and does not return to the core. This is referred as a cladding mode of propagation and may be excited light entering the cladding.
One example of a fiber optic coupler is an add/drop multiplexer (ADM). An ADM typically has an input fiber, an output fiber, a drop fiber, an add fiber, and a coupling region or junction. Within the coupling region, one of more fibers may have a grating made within them. The grating is a plurality of grating elements, equidistant from one another, where each grating element reflects a small portion of the light propagating through the core. The light that is reflected is narrowly centered about a central or resonant wavelength, and the cumulative effect of the plurality of gratings is to reflect the resonant wavelength into the drop fiber and remove it from the optical energy propagating within the core. These grating elements are typically referred to as fiber Bragg gratings (FBG).
The coupling region substantially couples all of the optical power from the input optical fiber into the output optical fiber, and the fiber grating will reflect the resonant frequency of the FBG into the xe2x80x9cdropxe2x80x9d fiber. Thus, the coupling region of an ADM is the area in which optical power is coupled between two or more fibers. These coupled fibers are placed proximal to one another so that optical communication and coupling between the fibers occurs. Coupling between the cores of each optical fiber and therefore a portion of the cladding must be removed or thinned to allow the optical coupling between the cores. The removal or thinning of the cladding is usually achieved by one of two methods: the first method is the use of a polished coupler and the second method is the use of a fused/taper coupler.
A polished coupler is one in which each of the two optical fibers is side-polished to remove a portion of the cladding. A FBG may be written on one of the optical fibers as an optical filter. The two fibers are then cemented together with an index matching material so that the side-polished portion of one fiber is held adjacent to, proximal to, and optically aligned with the side-polished portion of the other optical fiber.
Polished couplers suffer from several disadvantages however. The polishing of the fibers is a difficult technique to implement and the devices themselves are difficult to fabricate due to the small sizes of the polished regions. The small sizes make the alignment and adjustment of the coupler difficult. Also, the polished regions must be maintained in a parallel relationship to one another, over the coupler length, to avoid coupling problems. In addition, because of the fine alignment requirements and the need to maintain the polished regions parallel to one another, the coupling regions need to be held in a very stable manner. However, because each fiber will have a different composition from the other however slight, and the index matching cement is an entirely different material as well, a polished coupler is not environmentally stable. As each material expands and contracts at slightly different rates, the alignment of the fibers, the coupling efficiency, and the resonant frequency of any FBG written on a fiber will be changing. Finally, because only a portion of the cladding is removed or thinned, optical power will couple to the cladding within the coupling region and also that which borders on the polished region from the core. This will result in the excitation of cladding modes and an increase in insertion loss as discussed above.
A fused/taper coupler is one in which two or more optical fibers are wrapped together and heated. The two or more optical fibers are heated sufficiently to allow them to be drawn to form a central unified mass so that light on any input fiber is coupled through a thin cladding to the output fibers. The area that is heated and drawn is known as the xe2x80x9cwaistxe2x80x9d or xe2x80x9ctaperxe2x80x9d area because of the narrowed diameter when compared to the input and output fiber diameters. If at least one photosensitive fiber is used, then an FGB may be written on that optical fiber to act as an optical filter.
Fused/taper couplers suffer from several disadvantages however. By tapering the fiber, the optical power is driven from the core into the cladding. This occurs because the optical power is substantially carried within approximately three core diameters of the center of the core. Thus as optical power is driven into the core, cladding modes may be excited and optical power lost, increasing the insertion loss of the coupler. In addition, it is difficult to fabricate an ADM from a photosensitive optical fiber. It is thought that the small differences in the index of refraction between the photosensitive and photo-insensitive claddings is the cause of these problems.
What is needed in the art therefore is an ADM in which the excitation of cladding modes is reduced and in which the manufacture techniques are not as difficult as in the prior art.
The present invention provides a fused/taper fiber optic coupler that reduces the excitation of cladding modes and which is easier to produce than the prior art. A fused/taper fiber optic coupler comprises first and second optical fibers, each of the first and second optical fibers including a core section, an inner cladding layer coaxially disposed around the core section, and a outer cladding layer coaxially disposed around the inner cladding layer. The core section has a first index of refraction and the inner cladding layer has a second index of refraction that is less than the first index of refraction. The outer cladding also has an index of refraction that is substantially equal to the second index of refraction. First and second coupling regions are formed in the first and second optical fibers respectively, and each of the first and second coupling regions have the outer cladding layer circumferentially removed from the first and second optical fibers respectively. The first coupling region has a first length and the second coupling region has a second length. A tapered coupling junction formed from the first and second coupling regions helically intertwined together with a first helical pitch and length, wherein the first and second coupling regions are maintained substantially parallel and proximal to one another for optical coupling therebetween. A fiber Bragg grating is formed in the tapered coupling junction.